Signaling system



H. J. VENNES SIGNALING SYSTEM Filed July 16. 1918' z s: ets -shee-t 2 U U U 'i '11 U U Q b L mmww u E u H C -l u r u Ward/d J. V6/7/765.

Fatented Aug. M, 1923.

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IGNALING SYSTEM.

To all whomijtmay camera:

Be it known that I, HARALD J. Vnrmns, a citizen ,of the United States New York, in the county of State of New York, have invented certain new and useful Improvements in S1gnahng Systems, of clear, concise, and exact descr1pt1on.

This invention relates to signaling systems and more particularly to multiplex 1 carrier current systems in which signals are transmitted as variations or modulations of high fre uency carrier currents or oscillations, an a plurality of message carrier currents of different frequencies are transmitted simultaneously over the same trans-' mission line.

An object of this invention is to prov1d e means for preventing cross-talk in multiplex carrier current signaling systems.

A further object of this invention is to enable the use in a multiplex signaling system of carrier currents which are exact multiples of each other.

In multiplex carrier current systems, 1t 1s necessary to provide a receiving device which will respond only to carrier currents of a particular frequency range. It 18 also necessary where oscillations, locally or otherwise produced, are combined at the receiver with incoming modulated oscillations to prevent the production of any comblnation frequency oscillations which might otherwise be set up and which would appear as cross-talk in a channel responsive to such combination frequencies, The accomplishment of these objects presents considerabledifiiculty when unmodulated carrier frequencies are present in the line'and particularly where the unmodulated carrier frequencies are exact multlples of each other. Where amplifying devices are used the amplitude of such cross-talk frequency currents may be increased to such a degree thatsignaling is seriously interfered with. I

In ractice it is difficult to suppress all unmo ulated carrier frequency currents and their harmonics so as to keep the transmission line entirely free from them. Unmodulated carrier currents'of one frequency transmitted over the line may react with modulated oscillations of another carrier frequency to give modulated oscillations of residiiig at. New ork,

which the following is a full,

a third frequency whichwill pass the filter of one of the receivin channels and appear as cross-talk in that c annel.

' The present invention relates to an arrangement whereby a dissipation path. is I provided for these cross-talk or forelgn frequency currents without disturbin the selectivlty of the system, thereby elimlnating the interference resulting from these causes. This path is provided by grounding the electrlcal mid-point of one or more of the sections of each. filter, but preferably the mid-point of the end section of each wave filter electrically nearest to the si al transm ttmg or receiving apparatus. t is to be understood, however, that the invention is not to be limited to this specific connection to ground but includes any arrangement whlch compensates for electrical unbalance of a filter for currents of frequencies which it is desired to exclude.

In the drawing, Fig.' 1 illustrates the transmittmg circuits for a multiplexcarrier current telephone system; and-Fig. 2 shows the coresponding receiving circuits.

Referring to Fig. 1, three individual transmlttmg stations are shown, each comprislng a local signal circuit 1 a carrier current source 2, 3 or 4:, a modulator 5, an amplifier 6, and a band filter 7 The signal circuit 1 may contain a local source of current and a microphone, a source of low frequency oscillations and a key, or any other equivalent signaling arrangement, and ispreferably coupled to the input circuit of a balanced modulator 5 by a transformer 8. This modulator is preferably of a type disclosed in U. S. patent to Carson 1,343,306, issued June 15, 1920. Carrier current source 2 is connected to the input circuit of modulator 5 by a potentiometer 9. Modulated oscillations, which are set up during signaling periods in the output cirquencies ranging from the difference of the carrier and signaling frequencies to their sum, but is preferably arranged to pass only currents of one side frequency band as, for

example, the band ranging from carrier frequency down to the'difference of the carrier and signaling frequencies.

In Fig. 2 the transmlssion line 12 is shown connected to three receiving stations which correspond respectively to thetransmitting stations of Fig. 1. leach receiving station comprises a band filter 13 which selects currents of a band of frequencies of the range impressed upon the line by the corresponding transmitting station. Modulated carrier frequency oscillations of a particular band of frequencies are selected and are transmitted b each filter 13 to a detector 14, preferably 0 the balanced vacuum tube type, upon the input circuit of which OSClllations of carrier frequency are impressed in any desired manner as, for example, by a local source 15. The balanceddetector 14 acts like modulator 5 of Fig. 1 to suppress unmodulated oscillations of the carrier frequenc in its output circuit. The incoming modu ated oscillations and locally supplied unniodulated oscillations "are combined by the detector to give currents of their differ ence, or the original signal frequency, the output circuit of the detector. These signal currents are amplified by'an amplifier 16, preferably of the vacuum tube type, and are transmitted to receiving or indicating device 17 through an ultra filter 18 designed to cut off frequencies above the signal current range.

Connected to the electrical mid-points of both branches of a section 19 of filter 7, preferably the end section electrically near est to the transmitter, is a grounded conductor 20. -Currents of thcrange of frequencies for which filter 7 is designed will be conducted symmetrically by the filter which will, however, present considerable impedance to currents of other or foreign frequencies. Currents of other frequencies in effect appear to be conducted between one side of the filter and ground, the filter 'unbalance appearing to act as a difference in capacity between the two sides of the filter and earth or other conductor. Conductor 2O establishes a low impedance path between each side of the filter and ground for currents of frequencies outside of the transmission band. The unbalanced foreign frequency current will then be practically shortcircuited about a portion of the impedance path of the filter by this conductor. This does not materially change the impedance characteristics of the filter to a current of the frequency which the filter is designed to transmit. he value of the reactance between the grounded point and the ,sides of the filter isdete-rmined in each case for optimum operating characteristics.

A rounded conductor 21 is connectedto the efectrical mid-point of a section of filter 13, preferably the end section electrically emme nearest to-the detector, and serves toprotect the signal channel associated therewith from the effects of cross-talk currents. Since concircuit of detector 14 offers a high impedance tocurrents of these frequencies, their energy is dissipated in the low impedance grounded circuit.

The wave filters 7 are connected in series in the line 12, and each is of a type offering lowimpedance to currents of frequencies outside the range of transmission of its associated signaling channel but ofi'cring substantially infinlte impedance to currents within its transmission range. It becomes almost impossible to so place all the elements of .each filter that the capacity to ground for each unit will be balanced, and that the units will be balanced collectively as well as individually. The electrostatic capacity between the'windings of'the transformers connecting the various channels to line may also cause considerable unbalance. In order to transmission for the filter, and the input prevent short-circuiting of the filters, the

sections nearest thev transmission conductor must not be grounded. Groundmg the filtors in the manner described furnlshes a dissipation path for cross-talk currents, and in some instances may permit considerable unbalance under normal conditions without interfering with the selectivity ofthe system.

The invention has been disclosed as apfrequency carrier currents are employed for telephony and telegraphy. Obviously, however, it is in its broader aspects not limited to the use of modulated currents or even to the use of high frequency currents.

ing from thespirit and scope of the invention.

What is claimed is: 1. A line, and a plurality of wave filters connected in series in said line, each of said wave filters comprising an end section grounded at its mid-point.

2. A multiplex signaling system com-prising a line and a plurality of signal channels connected in series to saldline, eachof said channels including a filter having an end' section grounded at its electrical mid-point.

3. In combination, a line adapted to transmit currents of a plurality of bands of 'frequencies, individual-means connected in series in said line for selectively conducting each of said frequency bands of currents.

100 v plied to a system in-which modulated high aetitia and means to compensate for any electrical unbalance of said conducting means with respect to the two sides of said line whereby transmission thereby of current of any foreign frequency is prevented.

4:. A multiplex signaling system comprising a line and a plurality of channels connected in series relation thereto, each of said channels being designed to transmit currents of an individual range of frequencies, each of said channels having means to dissipate currents of any foreign frequency which tend to be transmitted thereby as a result of electrical unbalance of a channel with respect to the two sides of said line.

5. A signaling system comprising a line, a plurality of channels connected in series to said line,-each of said channels compris ing means to efliciently transfer currents of .a particular band of frequencies and to exclude currents of other frequencies, means connecting both sides of each channel to round, said connecting means oiiering high impedance to currents within the band of frequencies selected by the associated transfer means and offering low impedance to currents outside of said band.

6. A multiplex carrier current si naling system comprising a circuit and a p urality of wave filters connected to said circuit in series,- each of said filters having an end section grounded at its mid-point.

7.1m combinatiom'a line, and means to impress a plurality of currents of different frequencies upon said line, said means including a plurality of wave filters connected in series, each of said wave filters having an end section grounded at its mid-point.

8. A line, a plurality of transmitting devices, and means connecting each transmitting device to said line, said means being connected in series, andeach of said means conducting currents'of a difierent band of frequencies and comprising a wave filter having an end section grounded at its midpoint.

' 9. A signaling line, a plurality of transmitting stations, each of said. stations comprising a source of carrier frequency current, a source of low frequency current and a modulating device, and a wave filter connectin each of said stations to said line and a apted to transmit a band offcurrcnts of high frequency, said filters being all connected in series in said line and each having a section'grounded at its electrical midpoint.

10. In combination, a line, and means to select therefrom a plurality of currents of different frequencies, said means including a plurality of wave filters connected in series and each having an end section grounded at it mid-point.

11. A line, a plurality of receiving devices, and means arranged in series and connecting each of said receiving devices to said line, each of said connectin means conducting currents of a different and of frequencies and comprising a wave filter having an end section grounded at its midoint.

p 12. A signaling line, a plurality of receiving stations connected to said line, each of said receivin stations comprising a source of high requency currents and a detector, a wave filter connecting each of said receiving stations to said line all of said filters being connected in series and means for grounding the mid-point of an end section of each filter.

In witness whereof ll hereunto subscribe my name this 12th day of July, A. D., 1918.

HARALD 5. venues. f 

